Baking ovens are a very old art as are the more recent ovens using convection and/or conductive cooking surfaces upon which baked goods may be produced with reduced cooking time. Of particular focus within the art is the use a variety of cooking cycles in order to provide greater versatility and improved cooking performance. One common reason for varying cooking cycles was to utilize 110-volt service using elements whose wattage was the UL limit for a 110-volt service. That limit is to have no more than 13.5 steady state amperage draw and a total of 1550 watts. The UL wattage limits for 110-volt ovens, mandated an alternating cycle between upper and lower elements using full 13.5 amp cumulative capacity elements both top and bottom. The desire to provide a 110-volt cooking oven that could be plugged into a common household outlet combined with the need for higher wattage for adequate cooking performance precluded simultaneous upper and lower element operation due to the aforementioned current draw limitations. Until the present invention, alternating energizing of full 110-volt amperage capacity upper and lower cooking elements was necessary to afford adequate cooking performance and preheating times. An early example of this type of alternating cooking cycle oven was the two-stage microwave and radiant cooking technology employed by Raymond L. Dills, U.S. Pat. No. 4,188,520. Dills invention was an attempt to provide browning which microwave cooking alone cannot produce. The first cooking stage was solely microwave cooking and a second stage utilized an electric resistance coil to provide browning. This two-stage approach was further improved by Hurko et al., U.S. Pat. No. 4,242,554. Hurko's oven used a multiple alternation between microwave cooking and radiant cooking. This apparently afforded improved browning performance and purportedly created end products that are more consistent with conventional oven performance. The ideal behind the alternating cooking cycles in Hurko's invention was to create an oven whose current draw was reduced to meet ULI requirements for common household 110-volt service of no more than a total of 1550 watts and 13.5 steady state amperage draw. The advent of the small, portable convection oven includes Milton H. Farber, U.S. Pat. No. 3,828,760 wherein the cyclonic affect of heated fan driven convective air for rapid cooking was well demonstrated. Farber's convection oven, however, did not anticipate the conductive cooking possibility of utilizing a refractory, conductive cooking surface in conjunction with convection nor the thermal efficiencies as synergistically optimized in the present invention.
More recently, however, within the art, Victor R. Boddy, U.S. Pat. No. 5,695,668, utilized a similar two-stage cooking approach to Dills invention by introducing a portable conduction/convection oven with a conductive cooking slab. Boddy's oven afforded the same ability to produce foods with the rapidity of convective cooking but combined with the Hurko oven's capability to utilize a 110-volt circuit successfully. Boddy's two-stage cooking approach involved a preheating first stage to heat a high thermal mass refractory slab for conductive cooking. A conductive cooking second stage, wherein the lower element was de-energized and a convection fan and/or the upper element are activated during the actual cooking cycle was then employed. While Boddy's invention seems to provide a workable 110-volt service convection/conduction oven, there are several disadvantages inherent with that oven.
In order to maintain a stable cooking slab temperature, it is implied in Boddy's invention that a slab with a high thermal mass must be used in order to reduce the rate of heat loss. This is so because of Boddy's two-stage cooking process wherein the lower element that preheats this slab is turned off during the second stage wherein the upper elements are energized for cooking. In Boddy's invention, the preferred embodiment includes a refractory slab that is 1-1/2" thick and weighing 10 to 15 pounds. Since it is a thermal property that the greater the thermal-mass the slower the rate of heat gain and loss, the inverse is also understood (i.e. that the less the thermal-mass the more rapidly heat gain and loss occurs.) Further, it is common knowledge within the art that the consistency in the quality of baked goods produced by conductive cooking is directly related to the stability and even temperature of the cooking surface. By selectively heating the slab from beneath and then de-activating the element beneath the slab, the temperature of a slab of any thermal mass will inevitably decline. It is common knowledge within the art that stable conductive cooking surface temperatures are desirable so this occurrence in Boddy's slab is not desirable. The rate of slab temperature drop will be only partially offset by the small amount of radiant and convective heat provided from above that has contact with the bare upper slab surface. Most of this upper element source energy is absorbed by the foods being cooked. The rate of slab temperature drop is further influenced by the degree of temperature difference between the slab's temperature setting and the convective oven temperature setting. The rate of slab temperature drop is further accelerated if cold or frozen foods are placed on the slab. This is so since, in either situation whether heating air, slab or food, the rate of enthalpy is proportional to the temperature difference of the media involved. To reiterate, three primary problems exist with using Boddy's high thermal mass slab.
First, by using a high thermal mass conductive cooking slat, the rapid temperature drop desired when cooking puff pastries is not possible. Preparing puff pastry is a two stage cooking process. The first stage involves a high temperature of usually 425 to 450 degrees F. The second stage involves a lower temperature of usually 350 degrees F. Therefore, Boddy's high thermal mass slab with it's slow temperature drop is not feasible for cooking puff pastry. However, the use of a low thermal mass slab, as described il the present invention, is desirable.
Secondly, the use of a high-density slab makes the oven unnecessarily heavy. An improvement afforded in the present invention resolves this as will be elaborated below. The slab material costs and the cost to ship a heavier oven will cause the resulting retail price to be higher and/or profit margins to be smaller in order to compete in this market. Further, the difficulty in carrying and installing under counter or cabinet is also increased unnecessarily.
A third disadvantage of using a high-density slab is that preheat times are excessive making the oven thermally inefficient. Boddy's preferred embodiment indicated that initial preheating takes approximately one hour. While this preheating time is shorter than for some large brick pizza ovens in the prior art for commercial usage, it is unacceptable for a consumer oven. The deficiencies of the less relevant prior art are listed below.
Ishammar, U.S. Pat. No. 4,010,341 shows a hot air oven with an air circulating fan, a heater and circulation passages but also does not anticipate conductive cooking slab capabilities nor the thermal efficiencies as synergistically optimized in the present invention.
Vogt, U.S. Pat. No. 4,068,572 shows an apparatus for heating food using a horizontally displayed fan, but also does not anticipate conductive cooking slab capabilities nor the thermal efficiencies as synergistically optimized in the present invention.
Riccio, U.S. Pat. No. 5,605,092 while anticipating an oven with a stone covered bottom and supplemental heater, represents a heavy, high density, brick commercial type oven but does not anticipate the thermal efficiencies as synergistically optimized in the present invention.
Llodra, Jr. et al., U.S. Pat. No. 6,041,769 shows a portable brick oven with an arrangement of bricks and allows for convective/conductive heating. However this oven uses natural gas or propane, which is not as commonly available as the 110-volt electrical service used by the present invention nor does it anticipate the thermal efficiencies as synergistically optimized in the present invention.
McKee, et al., U.S. Pat. No. 6,060,701 shows a compact quick-cooking, conventional oven. While this oven utilizes a cyclonic vortex hot airflow convective cooking system, it represents a different air flow path than the present invention, and moreover, does not anticipate conductive cooking slab capabilities nor the thermal efficiencies as synergistically optimized in the present invention.
Beyond the improvements introduced to this field by the aforementioned prior art, as yet, the art has not seen a lightweight, thermally efficient portable 110-volt capable oven that affords effective cooking performance, using lower cumulative wattage elements without requiring an alternating cooking cycle with its resulting slab temperature fluctuations. The present invention solves these disadvantages while affording several new and significant improvements in energy efficiency, improved cooking versatility and end product quality. It should be noted that the present invention does not preclude the possibility of a 240-volt oven that incorporates the thermal efficiency improvements as applied to a 110-volt oven. As either oven would benefit from the thermal efficiency improvements of the present invention.
The current invention's 110-volt, preferred embodiment can cook all foods conventionally baked in small portable ovens in about 25% to 33% of the packaged oven time indicated, using just a total of 1400 watts. The use of (2) 350 watt elements for radiant and convective cooking within the cooking chamber and (2) 350 watt elements below the conductive cooking slab in conjunction with other collective thermal efficiency improvements elaborated below make this possible. Since the cumulative wattage of the present invention is below the 1550 watt UL, limit for a 110-volt service, simultaneous operation of the upper and lower elements is now possible without sacrificing cooking performance, exceptional cooking and preheating times and while affording exceptional energy savings.